Membrane glycoproteins behave as either carbohydrate or peptide antigens and occasionally express antigens constituted from specific structural elements present in both sugar and peptide moieties. Immune responsiveness to cell surface glycoproteins has not been studied systematically. We are characterizing the fine specificities of autoantibodies against glycoproteins of human erythrocytes from patients with altered immunologic states. In addition, we are preparing hybridomas that secrete monoclonal antibodies against various portions of the carohydrate and peptide moieties of human glycophorin A, the major sialoglycoprotein of human erythrocytes.